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Science: Methane may amplify climate change

2 June 1990

By JOHN GRIBBEN

WHEN the levels of methane in the atmosphere changed in the past, so,
too, did the climate, according to researchers who have examined an Antarctic
ice core. Methane, a greenhouse gas second in importance to carbon dioxide,
traps heat near the ground. It is produced mainly by bacteria which live
in swamps. At the moment, it is increasing rapidly because of burgeoning
agricultural activity, especially the spread of rice cultivation in paddies.

J. Chappellaz and his colleagues from the French Laboratory for Glaciology
and Geophysics, in St Martin d’Heres, near Grenoble, and from the Soviet
Arctic and Antarctic Research Institute, in Leningrad, have examined bubbles
of air trapped in an ice core. The core, drilled at the Vostok site in East
Antarctica, is 2083 metres long and covers the past 160,000 years (Nature,
vol 345, p 127).

The level of methane in the atmosphere changes due to natural fluctuations.
At the end of an ice age, for instance, both carbon dioxide and methane
increase their concentrations because of the increase in biological activity;
at the beginning of an ice age, they decrease. The effect of this is to
strengthen underlying temperature fluctuations that cause ice ages. These
fluctuations are caused by changes in the Earth’s orientation and orbit.

By examining the Vostok ice core, the Soviet-French team has found that
it takes several thousand years for the methane in the atmosphere to increase
from a low level to a high level and then decrease. The low levels occur
during an ice age, when the methane concentration is about 350 parts per
billion (ppb). This can increase to about 650 ppb during the warm spell
between ice ages, known as an interglacial. When the concentration is fluctuating
most rapidly, it changes at a rate of no more than 0.3 ppb per year.

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The amount of methane in the air has increased from about 700 ppb in
1700 to 1700 ppb today. This concentration is now more than double the highest
level reached before AD 1700.

Up to 1900, methane increased at a rate of 1.5 ppb per year; now, however,
it is rising at a rate of 17 ppb per year. This is far greater than any
natural fluctuation found in the ice core record, according to the researchers.

The Soviet-French team says that such a build-up of methane can produce
disproportionately strong greenhouse warming. At the end of an ice age,
the change from 350 to 650 ppb would on its own produce a warming of only
about 0.08 Degree C. But the extra methane in the stratosphere – the upper
atmosphere – will cause stratospheric water vapour to increase as the methane
oxidises. Water vapour is a powerful greenhouse gas. The extra water vapour
will contribute a further warming of 0.06 Degree C at the Earth’s surface,
so that the overall warming is almost 0.15 Degree C. This is about 30 per
cent of the direct effect of carbon dioxide (0.5 Degree C) for the same
increase in methane at the end of the ice age. Together, the build-up of
the two gases causes a warming of 0.65 Degree C, about a quarter of the
total change, at the end of an ice age.

The most likely reason for natural changes in methane concentration
in the past, says the team, is a change in the patterns of monsoon rain.
This increases the area covered by low-latitude wetlands when the world
warms, and these wetlands harbour bacteria that release methane.

Today, human activities are increasing the amount of methane in the
atmosphere at a rate that is 50 times as fast as at any other time in the
past 160,000 years, according to the French and Soviet researchers. ‘The
fundamental link between methane and climate variations indicated by the
Vostok record,’ they say, ‘suggests that the natural methane cycle may provide
a positive feedback in any future global warming.’ Just as it has amplified
ice age cycles in the past, it may amplify global warming.